The invention relates to thyristors of the type employing gate current generated by incident radiation penetrating the semiconductor material, such thyristors being generally known as light-fired or radiation responsive thyristors.
Light-fired thyristors are particularly useful in controlling high voltage circuits where a high degree of electrical isolation is important. In one typical type of installation, a light-fired thyristor is installed in a high voltage circuit and is triggered from a relatively distant location through conventional light fibers. The light fibers used are generally small, having diameters on the order of two or three thousandths of an inch, and are conventionally bundled together to form a larger light conduit. For thyristor triggering, one end of the light conduit is generally coupled directly to a light-fired thyristor.
Because the gate current available in light-fired thyristors is low, such thyristors must have relatively high gate sensitivity. As a result, light-fired thyristors tend to have short turn-on lines which significantly reduce the current handling capacity of the device during initial turn-on. Consequently, light-fired thyristors generally have relatively low di/dt ratings.
One technique for increasing the di/dt rating of a thyristor is described in U.S. patent application Ser. No. 907,931, filed May 22, 1978, assigned to the assignee of the present application. The thyristor described in application Ser. No. 907,931 provides for a plurality of separated light-responsive regions which produce multiple gating of the thyristor. If each of the separated light-responsive regions is illuminated simultaneously, the total turned-on area of the device is substantially increased. The di/dt rating of the device is thereby improved.
A significant problem encountered with coincident gating of a multigate light-fired thyristor is the coupling of the light fibers to specific points within the radiation receiving region of the device. Given the small size of the light fibers and of the thyristor itself, it is difficult and expensive to obtain accurate positioning of the fibers.